Varian 450-GC, User Manual

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Technical Tips
Varian, Inc. User Manual 450-GC Page: 461
TO BLEED OR NOT TO BLEED … THAT IS THE QUESTION
As GC and capillary column manufacturer Varian is regularly confronted with
customer queries with respect to rising baselines and baseline instability. These
effects are often attributed to column failures and unacceptable column bleed is
the diagnosis. But is this really the case? Here follows a short guide into more
effective troubleshooting on the baseline.
A rising baseline is part of many temperature-programmed analyses in gas
chromatography. The major factors contributing to this rise are:
1. Stationary phase bleed from the column.
2. Contamination in the Injector system.
3. Contamination of the detector system.
4. Carrier gas impurities.
Both the column and the Injector/detector system can play a role in baseline
rises and it is important to distinguish between the 2 sources.
Starting the troubleshooting exercise by immediately disconnecting the column
from Injector and/or detector can mean an unstable system for at least a couple
hours. It can be more effective to have a closer look at the baseline itself first.
The baseline below 250 °C
Improvements in column technology and stationary phase synthesis during the
last few years have resulted in highly temperature stable capillary columns.
Examples of these are the Varian Factor Four column range. These columns
only show some measurable bleed levels close to their maximum operating
temperature. This maximum temperature is often around 325 °C. The bleed
level of a capillary column roughly doubles with every 14 °C and it means that
already at oven temperatures 75 °C below the maximum temperature its bleed
level is virtually zero. Therefore, any significant baseline rise starting before 250
°C can safely be attributed to system contamination from medium to high
molecular weight residues. Most frequently these are originating from a
contaminated Injector. This contamination may use the column as a transport
device but the column is not the primary source. Major air leakage however,
may oxidize the liquid phase at temperatures lower than 250 °C and high
column bleed level can results of this.